1. Field of the Invention
This invention relates to refrigeration equipment and more particularly refers to a household refrigerator having a temperature-dependent control for switching-on a refrigeration unit and for defrosting the heat exchanger in the refrigeration chamber.
2. Description of the Prior Art
It is generally customary in refrigeration equipment to equip the regulator for controlling the cooling periods with only a single sensor. The latter measures either the temperature of the evaporator serving as a heat exchanger on the cold side, or the air temperature in the refrigeration chamber. Although the control of the refrigeration unit according to the evaporator temperature does permit proper defrosting of the evaporator, the consequence is an undesired dependence of the temperature in the refrigeration chamber on the room temperature prevailing at the point of installation of the refrigeration equipment. A temperature control which depends only on the air temperature in the refrigeration chamber has the disadvantage that proper defrosting of the evaporator is not ensured; for, in refrigeration equipment controlled only according to the air temperature in the refrigeration chamber, the danger exists that the refrigeration unit is switched-on again before the evaporator is completely defrosted. In this manner, the functioning of the refrigeration equipment which is controlled according to the air temperature is degraded by a gradually increasing deposit of frost and ice.
It has also been proposed to equip the control with two sensors, one of which monitors the air temperature in the refrigeration chamber and the other, the temperature of the evaporator and therefore ensures a complete defrosting of the latter. Thus, a control has become known, designed for example, as a bimetallic switch, which performs the function of the sensor for the air temperature and which is equipped with a heating resistor serving as the thermal restoring device for the bimetallic element. A second sensor which is likewise designed as a bimetallic switch is also arranged in the circuit of the control. This second sensor rests against the evaporator or heat exchanger of the refrigeration equipment and is adjusted so that its contacts are closed as soon as the evaporator is properly defrosted by a corresponding temperature rise due to natural incidence of heat or supplement heating. The heating resistor by the second heat sensor serving as the thermal restoring device for the bimetallic element of the control is therefore switched only when the evaporator is completely defrosted. A rise of the air temperature in the refrigeration equipment is simulated at the thermal bimetal of the control which constitutes the first heat sensor by the thereupon commencing heating-up of the heating resistor. This causes the control to switch, whereby the refrigeration unit starts up for a new cooling period.
It turned out, however, that the temperature control proper in this known arrangement exhibits a relatively large inertia because of the relatively high switch-on point due to manufacturing reasons and the easier adjustability, and therefore works merely as a time switch. Furthermore, the heating resistor of the thermal restoring device, which serves for compensating the high switch-on point, must first heat the thermal bimetal of the control, after defrosting, relatively strongly up to its switch-on value. In this manner, so much time is lost that the temperature in the refrigeration chamber can rise far beyond the set value.
It has been found to be another disadvantage of the known circuit that the thermal restoring required for compensating the relatively large switching interval of the bimetallic element is likewise greatly dependent on the ambient temperature prevailing at the point of installation of the refrigeration equipment, so that there also the switching range drifts if the ambient temperature changes.
The above described shortcomings have essentially had the result that refrigeration equipment provided with a control of this design never got beyond the experimental stage and never attained practical or economic importance.